The invention is in the field of analog-to-digital converters and relates specifically to converters of this type which have autozeroing circuits.
The autozeroing circuits in prior art analog-to-digital converters of the dual slope integrating type typically comprise a feedback connection operative prior to each dual slope measurement cycle to charge an input capacitor to an initial charge which reflects the amplifier and detector drift and offset, so that this drift and offset can e cancelled out during the immediately following dual slope measurement cycle. An example of such converter is the Model 1230 Digital Panel Meter manufactured by Weston Instruments, Inc., and additional examples are illustrated in Walton U.S. Pat. No. 3,566,397 and Howlett U.S. Pat. No. 3,581,305. The basic operating principles of dual slope integrating analog-to-digital converters are discussed in Hoeschele, D.F., Analog-to-Digital/Digital-to-Analog Conversion Techniques, Wiley, 1968, pp. 381-384.
Prior art converters of this type typically measure the voltage level of an input signal wih respect to the zero voltage line. It is difficult to provide for offsetting the input voltage in the analog portion of the converter, because the desired offsetting may be cancelled out by the autozeroing circuit in the same manner as the offset and drift of the integrator and the amplifier. Thus, it is difficult with prior art converters of this type to offset the input signal by a desired amount in the analog portion of the converter so as to provide a digital output which represents the level of the input voltage with respect to a nonzero reference line.